1. Field of the Invention
The invention relates to creating images by transmitting signals and sensing the effect of objects in the field of view on the signals.
2. Description of the Prior Art
The beamformed television method of U.S. Pat. No. 5,598,206 (January 1997) Bullis provided an underwater viewing system to display images sensed by a remotely operating part of the apparatus. This prior system was designed to operate as much like the popular video television camera as possible. This resulted in a system to determine scattering information in two angular dimensions and to form a picture from this information. This process was done at a repetition rate such that motion picture effects were achieved. This beamformed television system is very different from the usual conventional sonar where only one angular dimension is determined.
Beamforming is a signal processing operation applied in respect to arrays of acoustic transducers. Other types of transducers such as antennas can also be used. Receive beamforming is used to selectively receive propagating wave signals according to their angle of arrival. A receive beam can be electronically steered to change the direction from which to receive. Simultaneous receive beamforming means that there are multiple channels that selectively respond so as to receive propagating wave signals according to their respective angles of arrival. The response of these multiple channels is obtained by processing a single set of signals from the receiving transducers in the array. Transmit beamforming is used to control the direction of radiation relative to the transmitting array. It can also be electronically steered. Simultaneous transmit beamforming means that beams are emanated in multiple directions at the same time. To make this effective a coding system must be used to distinguish between the multiple transmit beams. Each such code is associated with a respective transmit beam direction. Use of these transmit beams requires that the codes be recognized and sorted after the reflected signals have been received. Where such simultaneous methods are used, the process of sequentially scanning through all the required directions is eliminated.
A visual like image can be generated if the locations of reflecting points in a field of view can be determined in two angular dimensions. Such reflecting point positions correspond, respectively, to an electronic memory location, or channel. The image data includes the strength of these reflections as a number stored in such a memory location. Such locations of reflecting points can be associated with respective pixels on a display device. Such pixels have positions on a display screen that are measured in the coordinates of the display screen. An effective imaging process is to associate the horizontal coordinate, or dimension, with one of the two angular dimensions and to associate the vertical coordinate, or dimension, with the other of the two angular dimensions. The strengths of the respective reflections are then electronically plotted on the screen. The brightness of such plotted points represents the strength of the reflection from that point. This is the way optical systems such as television systems generally work. It is also the way the human eye works.
In U.S. Pat. No. 5,598,206 (January 1997) Bullis, disclosed systems were built around a concept of orthogonal linear arrays and simultaneous beamforming techniques. Such systems are attractive for determining such positions in angular dimensions. Herein, a large number of transmitting elements were arranged in a line and a large number of receiving elements were arranged in a perpendicular line. The array beamforming operations involved simultaneous transmit beamforming and simultaneous receive beamforming so that previously mentioned rapid coverage of the field of view was accomplished. This coverage was accomplished in the time interval required for a single burst transmission and a single round trip propagation interval.
The two dimensional angular locations of all reflecting points can be determined by a combination of the transmit beamforming system and the receive beamforming system. A number, N, of transmit beams are formed in the system. For each of these transmit beams, a number, M, of receive beams are formed. This results in N.times.M channels. The amplitude of the signal in each such channel represents the strength of reflection from the region where the beams overlap. The result is the image data that drives N.times.M pixels according to the associated reflection strength. The elements in both arrays must be appropriately arranged and the two arrays must be appropriately oriented so as to provide a complete, unambiguous set of such image data. Such system configurations must be designed to be practical and economical in the particular application.
The prior patent U.S. Pat. No. 5,598,206 (January 1997) Bullis addressed underwater imaging and also referred to radar, and medical imaging applications as other possible applications. Reference was made to general application possibilities which arise where other forms of radiating wave signals are available. Systems built around the concept of orthogonal linear arrays were found to be attractive. A large number of transmitting elements were arranged in a line and a large number of receiving elements were arranged in a perpendicular line. The use of both simultaneous transmit beamforming and simultaneous receive beamforming showed great promise because complete coverage of the field of view was accomplished as a result of a single burst transmission, thus achieving a nearly instantaneous scan over two dimensions. A number of important distinguishing features were necessary to make these basic concepts into an effective system.
In addition to underwater imaging use, applications to medical ultrasound imaging are especially interesting. In this field, state of the art linear array systems exist that use a single array for both transmitting and receiving. A single transmit beam is formed in a direction and a single receive beam is formed in that same direction to receive the reflections from the transmit beam. The angle dimension of the resulting picture is simply the angle of the beam. The range dimension of the resulting picture is obtained from the arrival time of a reflected signal. This process is repeated sequentially to accomplish scanning of the object of examination. This complete scan can be done rapidly to enable a full frame to be obtained and displayed in a short time. Repetition of this frame provides motion picture like effects. However, the display is range versus angle so the format is not consistent with normal way things are seen by the human eye. This is a very different type of display than that discussed in U.S. Pat. No. 5,598,206 (January 1997) Bullis which would provide an elevation angle versus azimuth angle display to enable a more visual like image presentation.
A direct application of U.S. Pat. No. 5,598,206 (January 1997) Bullis to medical ultrasound imaging has been examined. Such a direct application would allow the viewer to see into the human body but the many intervening layers will all show as overlays of each other. It is important to be able to discriminate. Therefore the range resolution issue becomes significant. In U.S. Pat. No. 5,598,206 (January 1997) Bullis, methods to selectively view range zones based on depth of field effects in the near field were disclosed. This method of selectivity is not as precise as desired in some circumstances. Range resolution inherent in the properties of waveforms used for coding was also disclosed in U.S. Pat. No. 5,598,206 (January 1997) Bullis. Codes discussed were simple, single frequency pulses but other codes were indicated as an option. Codes such as pseudo random noise (PRN) are well known codes to use for improving range resolution in radar and sonar. Use of such codes will give range resolution inherent to the code properties as a result of code channel formation as disclosed. Such codes provide full resolution of the range dimension and this enables selective range zone viewing. However, use of codes to give fine grain range resolution leads to a difficult trade off between processing time and range resolution. Where the range resolution is small, the appropriate method for beamforming is a time delay method. Time delay methods are very desirable except that they require lengthy signal processing operations. Because of this, the methods in U.S. Pat. No. 5,598,206 (January 1997) Bullis have practical drawbacks.
Not all internal parts of the human body are stationary. Sensing motion is an important capability in medical ultrasound systems. The invention U.S. Pat. No. 5,598,206 (January 1997) Bullis provided for selectively viewing moving scattering centers. The resulting image signals enabled viewing of an image representing all particles having a particular radial velocity. However, this invention fails to make a full field of view image of the radial velocity of the individual scattering centers.
From another field, there are commercial laboratory instruments that use a stepped-chirp signal with FFT (fast Fourier Transform) processing to resolve the time dimension. An example is the Hewlett Packard 8510 Network Analyzer family of instruments. There are also laboratory instruments that use a gated-tone method to resolve the time dimension. Flam and Russel Co. produced such a product as did Scientific Atlanta Co. In the field of radar cross section measurement it is known to use such instruments in combination to measure low level signals. In some cases radar cross section imaging systems have been assembled which produce images having dimensions of range and cross range. Such systems used controlled target rotation and synthetic aperture processing to resolve the cross range dimension. Such systems did not use of transducer array methods to resolve the angular dimensions. They did not produce visual like images.
In still another field there are known sonars that utilize continuous transmission frequency modulation to determine range to targets by frequency of an audible tone.
The invention U.S. Pat. No. 5,598,206 (January 1997) Bullis provided for basic features to yield acceptable image quality. These included disclosure of a special linearity requirement in the transmitting system such that times when the transducer output saturates are infrequent. Allowing occasional saturation adds to the clutter in the image.
The following list of objects and advantages will make apparent the benefits of a very efficient, full three dimensional system having fine grain resolution in all three dimensions.
The patents referenced in this document are incorporated by reference. In case of conflict, the present document takes precedence in all respects.